1. Field of the Invention
The present invention relates to a switched attenuator for selectively attenuating high frequency signals. More particularly, the present invention relates to a switched attenuator which maintains linearity across a wide frequency range.
2. Description of the Related Art
Due to parasitic capacitance, a Gallium-Arsenide (GaAs) FET will present a decreasing isolation as a function of frequency when turned off. This is illustrated by the solid line in FIG. 1 which shows a typical S21 isolation response of a GaAs FET in an off state. FIG. 1 also shows a dotted line representing the typical insertion loss of a GaAs FET in an on state.
Due to the isolation characteristics, just one GaAs FET is not typically used in a switched 20 dB pad which is intended to function up to 3 GHz. In the on state, a single GaAs FET can have an insertion loss lower than xe2x88x921 dB at 3 GHz. Such a low insertion loss, as illustrated in FIG. 1, which is associated with the low on state return loss, is not identified as being provided in FET attenuators advertised by current manufacturers. FIG. 2 shows a prior art switched attenuator which includes two GaAs FET""s to form the switches 2 and 4.
In accordance with the present invention, a phase compensated switched attenuation device is provided for attenuating high frequency signals. In one embodiment, a through-path GaAs FET is coupled between input and output ports for switching the device between a through state and an attenuation state. First and second isolation FET""s are coupled to the input port and output port, respectively for isolating the through-path FET from a pad coupled to the isolation FET""s. A resistor or a series combination of a resistor and capacitor can be coupled to the first and second isolation FET""s in parallel with the pad. Resistors are provided for coupling gates of the FET""s to a plurality of voltage sources. A device in accordance with one embodiment of the present invention can maintain an insertion loss of less than 1 dB for frequencies up to 3 GHz.